This invention relates to a drive mechanism for driving a working unit, movable in an oscillating manner, of electric products of personal use, particularly dry shavers.
Dry shavers which cut off beard hairs with a reciprocating cutter block are known examples of household appliances with an oscillating working unit. Oscillating dry shavers are traditionally driven by a rotary electric motor, for example a direct current motor. The rotary motion of the motor shaft is transformed into a linear oscillatory motion through suitable eccentric gearing.
Unlike gear solutions of this type, the linear oscillatory motion of the working unit may also be generated directly by a direct drive. This obviates the need to provide the eccentric gearing for transforming the rotary motor motion into a linear oscillation.
One form of a quasi linear direct drive is, for example, the so-called oscillating armature motor (cf., for example, DE 31 39 523 A1). Such oscillating armature motors are disadvantageous, however, on the one hand in that they can only be operated with a 220 volt line voltage, which makes them unsuitable for battery-powered appliances, and on the other hand in that they produce very large stray magnetic fields.
A vibratory drive for dry shavers is also known in which a rotor arrangement capable of performing rotary oscillations is connected to the cutter block by a cable control via deflection pulleys and drives said cutter block (cf. DE 41 17 225 A1) This vibratory drive which is operated with line voltage has a relatively low level of efficiency and is unsatisfactory with regard to its size and dimensions. The yieldability of the drive mechanism limits its efficiency.
A linear permanent magnet motor is also known as a direct drive for a dry shaver, in which a motion unit with permanent magnets is suspended from leaf springs in the manner of a suspension bridge above a stator with an exciter winding and drives a coupled cutter block (cf. WO 95/26261). A sensor monitors the speed of the motion unit, in dependence on which a controller modifies the electric power fed to the motor in such a way that the amplitude of oscillation of the motion unit is maintained constant. However, this known direct drive for dry shavers is complicated and less compact in design. The precision necessary for a good level of efficiency can only be achieved with a great outlay.
It is therefore an object of the present invention to provide an improved drive mechanism of the type initially referred to which avoids the disadvantages of known drives. In particular it is desirable for the drive mechanism to be of simple design and achieve stable operation with a high level of efficiency.
This object is accomplished according to the invention by a drive mechanism of the type initially referred to, with a permanent magnet motor which has a stator with an exciter winding and a rotor with at least one permanent magnet positioned in the stator field and rotatably mounted on a shaft, with a reset device for returning the rotor to a position of rest in such a way as to enable the rotor to oscillate about the position of rest, with a coupling device for coupling the rotor to the working unit, wherein the coupling device is connected to the rotor in spaced relationship to the rotor shaft, with a detecting device for detecting the oscillatory motion of the rotor, and with an actuating unit for actuating the permanent magnet motor in response to the detected oscillatory motion, provision being made for frequency control elements for actuating the permanent magnet motor with a frequency corresponding approximately to the resonant frequency of the rotor.
The rotary bearing arrangement of the permanent magnetic motion unit configured as a rotor affords the advantages of high stability and rigidity. Unlike a spring suspension in the manner of a suspension bridge in the case of the linear motor, the rotor with a rotary oscillating motor does not undergo any rocking motion. The rotary oscillating motor leads in a dry shaver to reduced foil wear and permits a smaller distance to be maintained between the foil and the cutter blocks. Advantageously, the air gap between the rotor and the stator can be very small, in particular around0.5 mm or less. On account of the rotary bearing arrangement there is no risk of contact between the rotor and the stator. This results in a high level of efficiency.
The working unit is not coupled to the rotor shaft but is eccentrically connected directly to the body of the rotor by the coupling device. Favorable leverage ratios and a high level of rigidity are thus achieved. The rigid and direct transmission of the rotor""s rotary motion to the working unit displays a high level of efficiency, particularly for oscillations of high frequency far in excess of the line frequency, and is the reason for such high-frequency oscillations being at all possible.
The actuating unit, to which the oscillatory motion of the rotor is fed back via the detecting device connected thereto, actuates the exciter winding of the stator such that the rotor oscillates with a frequency corresponding approximately to the resonant frequency of the system comprised of the rotor and the reset device. Operation at resonance achieves a very high degree of efficiency. Unlike weighty line-operated appliances with a low level of efficiency it is possible, on account of the actuating unit, to achieve very high operating frequencies in the range of 104 minxe2x88x921, thus enabling the shaving time to be shortened. In conjunction with the rigid coupling of the working unit it is possible to compensate for load peaks, such as occur during shaving, very quickly, particularly from one half cycle to the next. This leads to a higher quality and speed of processing. As a result of the high level of efficiency the drive mechanism can be built to small dimensions. Together with the motion element""s configuration as a rotor it is thus possible to achieve a highly compact design.
According to a preferred embodiment of the invention the rotor has various sections made of various materials. In particular the rotor may have an inner lying core made of a soft magnetic material of high permeability and poles made of a permanent magnetic material. Conversely, it is also possible for the rotor to have an inner lying core of permanent magnetic material and poles made of a soft magnetic material of high permeability. On account of smaller stray losses of the poles made of permanent magnets, the air-gap field and hence the output power and efficiency of the drive are greater than with the second arrangement. Furthermore, the inductance of the motor viewed from the terminals of the exciter winding is smaller. Consequently the current is allowed to increase more quickly, which again raises the output power. The rotor is advantageously configured so that the inductance is independent of the rotor""s position. This simplifies the electronic actuation of the motor. It is also an advantage for the rotor shaft to be worked into the soft magnetic material and not into the magnet material, which is relatively hard to machine. This simplifies production substantially.
It is also possible for the rotor to be comprised completely of permanent magnetic material. In this case the rotor shaft is preferably not made of permanent magnetic material. The embodiment in which the rotor is made completely of permanent magnetic material is particularly advantageous with regard to the level of efficiency. However, the previously mentioned embodiment has advantages with regard to production and machining.
According to a preferred embodiment of the invention the reset device is connected to the rotor in spaced relationship to the rotor shaft. Hence the reset device does not engage the rotor shaft. Precise oscillation of the rotor is effected using a lever arm acting on the rotor. It will be understood that the reset device can also act on the working unit. The resetting force is preferably transferred to the rotor via the coupling device which connects the working unit to the rotor. Hence the reset device is operatively associated with the coupling device.
Resetting the rotor from its displaced positions to its position of rest may be effected in a variety of ways. By way of example, the spring-type resetting force can be generated by magnetic fields, for example resetting magnets can be attached to the rotor to act as a magnetic resetting device. Magnetic resetting forces can also be used in combination with other reset devices. Preferably, however, resetting the rotor is effected by a mechanical spring device. Leaf springs or helical springs can be used for this purpose. The reset device preferably includes a return spring with a fastening section for fastening to the rotor and a coil section surrounding the fastening section. In this way it is possible to obtain a particularly compact and space-saving arrangement of the reset device. The reset device is preferably arranged on the end of the motor, in particular it is possible for the fastening section to be connected to the rotor at its end.
The return spring is preferably of a plane configuration in order for the reset device to take up only little structural space. The fastening section and the coil section surrounding it extend in one plane. The thickness of the return spring is determined by the thickness of the material.
A particularly favorable configuration of the return spring is obtained by constructing the fastening section in an essentially Z-shaped configuration with a pair of engagement sections lying opposite and essentially parallel to each other, and by providing the coil section with a pair of coil arms extending outwardly in spiral shape from the engagement sections. The engagement sections are designed to bend resiliently such that the relative distance of the engagement sections is variable. Hence the return spring can be straddled in place with its engagement sections. The spring body of the return spring consisting preferably of spring steel can be constructed with various cross-sections, but preferably it is rectangular.
According to an advantageous embodiment of the invention the rotor is arranged standing, i.e., with its end facing the working unit. The coupling device is arranged on one end of the motor. It is thus possible to obtain a very compact design. The working unit can be arranged in very close proximity to the rotor without being hindered by the stator. The freedom of stator design remains unrestricted.
According to a further advantageous embodiment of the invention it is also possible for the rotor to be arranged lying, i.e., the peripheral side of the rotor faces the working unit. The coupling device is preferably arranged on the peripheral side of the rotor. This arrangement has advantages with regard to the bearing of the rotor. The end of the rotor does not need to be accessible for the coupling device, resulting in greater freedom of design for supporting the rotor shaft.
In a further aspect of the invention the coupling device is constructed to be unyielding in the direction of movement of the working unit and with play in a direction perpendicular to the direction of movement of the working unit. Hence the coupling device is yielding vertically to the direction of movement of the working unit while being rigid in this direction of movement. The driving motion of the rotor is transferred to the working unit directly, without delay and with precision. Yieldability perpendicular to the direction of movement permits a corresponding component motion resulting from the rotation of the rotor to be compensated.
To obtain a simple design of the coupling train with a high level of stiffness, the coupling device is preferably made of only two parts. A first coupling element which is fixedly connected to the rotor, and a second coupling element which is fixedly connected to the working unit, are in direct engagement with each other, the two coupling elements being preferably rigid. Providing the coupling device with only two rigid elements enables direct transmission of the drive motion without delay and permits high frequencies of oscillation. Yieldability, as exists with known coupling devices operating with cable controls, is avoided.
A particularly simple and rigid construction is achieved when one coupling element is a pin and the other coupling element a cutout, particularly an elongate slot. It will be understood that the arrangement of the two coupling elements is interchangeable. However, the pin is preferably disposed on the rotor and the cutout on the working unit. This simplifies the assembly of the reset device, enabling it to be mounted independently of the working unit, in addition to facilitating the handling of the working unit during cleaning or the like. In cases where the coupling device is arranged on the end of the rotor, the pin preferably extends parallel to the rotor shaft. In cases where the arrangement is on the peripheral side of the rotor, the pin preferably extends perpendicularly to the rotor shaft.
In a further aspect of the invention provision is made for a pair of coupling devices which are associated with various working units and arranged on opposing sides of the rotor shaft in such a way that the working units are driven in counter-running direction. The counter-running motion of the working units effects a reduction of vibrations, for example, on parts of the housing, the inertias of the working units counterbalancing each other. The arrangement of the coupling devices on opposing sides of the rotor shaft is advantageous particularly in connection with the standing arrangement of the rotor. The corresponding coupling elements are rigidly connected to the rotor. The coupling devices are rigidly interconnected by the rotor. The working units oscillate in exactly opposite directions. They oscillate with the same frequency of oscillation even under the imposition of different loads to the working units. Actuation of the motor is thereby simplified.
In accordance with yet another advantageous configuration of the invention provision may be made for several rotors, each of which is associated with one working unit. Hence each working unit is driven by a rotor of its own. In this arrangement each rotor may be optimally adapted to the corresponding working unit. Preferably the rotors have a common shaft. This simplifies the structural design of the drive mechanism. It will be understood that the rotors may be of the same sign and oscillate in phase. However, the rotors preferably have different directions of magnetization such that they oscillate in opposite directions to each other. This reduces vibrations to a large extent. Not only the inertias of the working units but also the inertias of the rotors counterbalance each other.
The oscillatory motion of the rotor is detectable by way of various quantities of motion. To control the energy supplied to the motor it is possible, for example, to detect the rotor displacement or the resetting force of the reset device acting on the rotor and to feed these quantities back to the actuating unit. However, the rotor speed is preferably detected as the quantity of motion characteristic of the rotor oscillation and fed back to the actuating unit for actuating the motor.
It will be understood that the oscillatory motion is detectable by various sensors, e.g., Hall probes or optical sensors.
An advantageous configuration of the invention resides in that the detecting device comprises a sensor device for detecting a stray flux created by the permanent magnet of the rotor. Detecting the rotor""s oscillatory motion is thus performed without the help of any auxiliary magnets. The rotor motion is detected by means of the magnetic flux originating directly from the permanent magnet of the rotor. The permanent magnets form part of the detecting device.
The detecting device is preferably integrated in the stator, a being in particular arranged on a pole shoe of the stator. In this arrangement the sensor coil may simply be wound around a tooth of a pole shoe of the stator. The advantage of integrating the detecting device in the stator is that it enables a space-saving, compact design of the drive mechanism to be achieved.
To enhance the accuracy of detection of the oscillatory motion of the rotor, the detecting device includes a compensating device to compensate for an error content in the signal of the sensor device. In addition to the flux of the permanent magnets it is possible for the magnetic flux of the exciter winding proportional to the motor current to induce a voltage in the sensor device whose fundamental wave is contained likewise in the output signal of the sensor device. The compensating device preferably has a current sensor for detecting a motor current and a subtraction unit for subtracting a current sensor signal proportional to the motor current from the signal of the sensor device. Hence the detecting device provides, independently of the operating state of the motor, an output signal which is a measure of the rotor speed. The motor can thus be controlled so that for different loads the motor runs at its resonant frequency with a constant cutter speed.
The process of detecting the oscillatory motion of the rotor may also be performed indirectly. According to another embodiment of the invention the drive mechanism has no oscillation sensor. A signal proportional to the motor current is fed back to the actuating unit. A movement of the permanent magnetic rotor has a direct effect on the motor current via the voltage induced in the stator coil. The motor current can be detected with a suitable current sensor and this signal is fed back instead of the signal of a motion sensor to the actuating unit in order to actuate the motor. Using a second signal, which is a measure of the fundamental wave amplitude of the terminal voltage driving the motor, together with the motor current signal it is possible to draw a conclusion about the voltage induced in the motor. This voltage is proportional to the rotor speed, thus enabling this quantity to be controlled at different loads.
To effect the desired oscillation of the rotor the actuating unit controls the electric energy fed to the motor in response to the rotor""s oscillatory motion by means of a control device. The control device includes a peak value detector for detecting a signal peak value from the detecting device and controls the energy fed to the motor in response to the detected peak value. Reducing the oscillation signals to one value simplifies the actuation of the exciter winding without noticeably impairing the accuracy. The detected peak value can be compared to a setpoint value and the energy fed to the motor can be controlled in response to the difference.
To exert an influence on the rotor""s oscillatory motion it is possible to modify the energy supply to the motor in various ways. The actuating unit preferably has a signal generator for actuating the motor with bipolar voltage pulses, wherein provision is made for a modulating device for modulation of the voltage pulses as a function of the oscillatory motion of the rotor. According to a first configuration of the invention it is possible to exert an influence on the amplitude of oscillation of the rotor by effecting a phase modulation of the bipolar pulses, i.e., a process referred to as pulse-phase modulation. A displacement of the bipolar voltage pulses relative to the oscillation cycle of the rotor can be performed by means of a phase shifter controllable by the control device. According to a further configuration of the invention it is possible for the electric power fed to the drive mechanism to be controlled by pulse-amplitude modulation. In this process the amplitude of the voltage pulses is modified, thereby controlling the quantity of electric energy in a voltage pulse. In particular it is also possible for the output signal of the signal generator to be modulated in such a way that the bipolar pulses for actuating the motor are affected in their pulse duration by the controller. Hence with a process referred to as pulse-duration modulation the duration of the voltage pulse is modified, thus effecting a variation of the power fed to the motor. It will be understood that it is possible to combine the various forms of modulation with each other. However, using a single form of modulation simplifies the electronic control devices and achieves a sufficient level of accuracy.
According to a preferred configuration of the invention the signal generator is configured such that the voltage pulses are generated in a constant phase relationship to the rotor""s oscillatory motion. If pulse-duration modulation is performed to control the amount of energy supplied, then contrary to pulse-duration modulations customary for similar applications, the voltage pulses are not generated at a fixed starting instant involving modification of only the end of the voltage pulses, but instead the drive pulses are widened or narrowed symmetrically on both sides. A rigid phase coupling between the fed back sensor signal, which is characteristic of the rotor""s oscillatory motion, and the fundamental wave of the pulse-duration-modulated drive pulses is thus obtained even with a changed pulse duty factor of the pulse-duration modulation. A particularly high level of efficiency is thereby achievable. The permanent magnet motor is operated exactly in resonance.
At least one variable direct voltage level is preferably generated by the control device and compared, by means of suitable comparators, to a triangular signal generated by the signal generator synchronously with the oscillation of the rotor, the pulse duration of the voltage pulses being fixed according to the points of intersection of the triangular signal with the direct voltage level.
It will be understood that the process of actuating the motor with the bipolar voltage pulses may take place in a variety of ways. According to one embodiment of the invention the frequency control elements include an oscillator for actuating the rotor with a predetermined frequency. This predetermined frequency preferably corresponds approximately to the resonant frequency of the rotor in conjunction with the reset device.
To achieve a particularly high level of efficiency and to effect operation in resonance even on variations of the resonant frequency, the frequency control elements according to a further embodiment have a self-oscillating feedback loop. This loop may include in particular a phase shifter for the signal of the detecting device, the signal generator for determining the sequence of the bipolar voltage pulses, a power stage for actuating the motor, and the control device for controlling the electric energy fed to the motor. The great advantage of actuating the motor in a self-oscillating loop is that the actuating frequency is invariably maintained exactly resonant, i.e., independently of a resonant frequency shift, for example due to loading of the drive mechanism with a speed-dependent force. Dips in the amplitude of oscillation due to the actuating system falling out of step are avoided.
An important aspect, particularly for dry shavers, is to achieve a compact arrangement of the drive mechanism. The shavers should be on the whole small and handy while, on the other hand, space is needed sufficient to receive an energy store such as a storage battery. Advantageously designed dry shavers have a pivot head which can be pivoted relative to a shaver housing and carries at least one cutter unit adapted to be driven by the drive mechanism. As a result of the pivot head it is possible to achieve an optimal placement of the cutter unit against the facial contour to be shaved. According to a further aspect of the invention a dry shaver of the type initially referred to has the drive mechanism integrated in the pivot head.
Hence the drive mechanism is pivotal together with the pivot head relative to the shaver housing. A particular advantage of this arrangement is that it enables greater mobility of the pivot head. The coupling of the drive mechanism to the at least one cutter unit needs to display no degree of freedom or mobility because relative motion between the cutter unit, which is pivotal with the pivot head, and the drive mechanism is unnecessary. The drive mechanism pivots with the pivot head.
According to a preferred configuration of the invention the drive mechanism with the rotor shaft is arranged standing, in particular the pivot axis of the pivot head can be vertically intersected by the rotor shaft. In a position of rest of the pivot head, i.e., in a position that is not pivoted out of the shaver housing, the rotor shaft can extend parallel to the longitudinal axis of the shaver housing. Several parallel cutter units can advantageously be driven by a rotor in opposite directions and be rigidly coupled together.
A particularly low-vibration configuration is obtainable by arranging the rotor shaft in a lying position.